glCreateShader和glCreateProgram在SDL 2 IOS中返回0

我正在尝试使用c++和SDL2在OpenGL ES 2.x中渲染一个简单的三角形。但是glCreateShader和glCreatProgram返回零。Bellow是我正在使用的代码

#include "SDL.h"
#include <OpenGLES/ES2/gl.h>
#include <OpenGLES/ES2/glext.h>
#include <iostream>
#include <string>
#include <memory>
using namespace std;
GLuint programObject;
class Graphics
{
private:
    SDL_Window* _window;
public:
    Graphics(SDL_Window* window)
    {
        _window = window;
    }
    void update()
    {
        glClearColor(255.0f, 0.0f, 255.0f, 1);
//        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
        GLfloat vVertices[] = {
            0.0f, 0.5f, 0.0f,
            -0.5f, -0.5f, 0.0f,
            0.5f, -0.5f, 0.0f
        };
        glViewport (0, 0, 320, 480);
        glColorMask (GL_TRUE, GL_TRUE, GL_TRUE, GL_FALSE);
        glClear (GL_COLOR_BUFFER_BIT);
        glUseProgram (programObject);
        glVertexAttribPointer (0, 3, GL_FLOAT, GL_FALSE, 0, vVertices);
        glEnableVertexAttribArray (0);
        glDrawArrays (GL_TRIANGLES, 0, 3);
        SDL_GL_SwapWindow(_window);
    }
};
void UpdateFrame(void* param)
{
    Graphics* graphics = (Graphics*)param;
    graphics->update();
}
///
// Create a shader object, load the shader source, and
// compile the shader.
//
GLuint LoadShader(GLenum type, const GLchar *shaderSrc)
{
    GLuint shader;
    GLint compiled;
    // Create the shader object
    shader = glCreateShader(type);
    if(shader == 0)
    {
        cerr << "Could not create OpenGL shader " << endl;
        return 0;
    }
    // Load the shader source
    glShaderSource(shader, 1, &shaderSrc, NULL);
    // Compile the shader
    glCompileShader(shader);
    // Check the compile status
    glGetShaderiv(shader, GL_COMPILE_STATUS, &compiled);
    if(!compiled)
    {
        GLint infoLen = 0;
        glGetShaderiv(shader, GL_INFO_LOG_LENGTH, &infoLen);
        if(infoLen > 1)
        {
            char* infoLog = (char*)malloc(sizeof(char) * infoLen);
            glGetShaderInfoLog(shader, infoLen, NULL, infoLog);
            SDL_Log("Error compiling shader:n%sn", infoLog);
            free(infoLog);
        }
        glDeleteShader(shader);
        return 0;
    }
    return shader;
}
///
// Initialize the shader and program object
//
int Init()
{
    const char vertexShaderString[] =
    "attribute vec4 vPosition;    n"
    "void main()                  n"
    "{                            n"
    "   gl_Position = vPosition;  n"
    "}                            n";
    const char fragmentShaderString[] =
    "precision mediump float;n"
    "void main()                                  n"
    "{                                            n"
    "  gl_FragColor = vec4 ( 1.0, 0.0, 0.0, 1.0 );n"
    "}                                            n";
    GLuint vertexShader;
    GLuint fragmentShader;
    GLint linked;
    vertexShader = LoadShader (GL_VERTEX_SHADER, vertexShaderString);
    fragmentShader = LoadShader (GL_FRAGMENT_SHADER, fragmentShaderString);
    programObject = glCreateProgram ();
    if (programObject == 0) {
        cerr << "Could not create OpenGL program" << endl;
        return 0;
    }
    glAttachShader (programObject, vertexShader);
    glAttachShader (programObject, fragmentShader);
    glBindAttribLocation (programObject, 0, "vPosition");
    glLinkProgram (programObject);
    glGetProgramiv (programObject, GL_LINK_STATUS, &linked);
    if (!linked) {
        GLint infoLen = 0;
        glGetProgramiv (programObject, GL_INFO_LOG_LENGTH, &infoLen);
        if (infoLen > 1) {
            char* infoLog = (char*) malloc (sizeof (char) * infoLen);
            glGetProgramInfoLog (programObject, infoLen, NULL, infoLog);
            cerr << "Error linking program: " << infoLog << endl;
            free (infoLog);
        }
        glDeleteProgram (programObject);
        return 0;
    }
    glClearColor (0.0f, 0.0f, 0.0f, 1.0f);
    return true;
}
int EventFilter(void* userdata, SDL_Event* event)
{
    switch (event->type)
    {
        case SDL_FINGERMOTION:
            SDL_Log("Finger Motion");
            return 0;
        case SDL_FINGERDOWN:
            SDL_Log("Finger Down");
            return 0;
        case SDL_FINGERUP:
            SDL_Log("Finger Up");
            return 0;
    }
    return 1;
}

int main(int argc, char** argv)
{
    /* initialize SDL */
    if (SDL_Init(SDL_INIT_VIDEO) < 0)
    {
        printf("Could not initialize SDLn");
        return 1;
    }
    SDL_DisplayMode displayMode;
    SDL_GetDesktopDisplayMode(0, &displayMode);
    /* create window and renderer */
    SDL_Window* window = SDL_CreateWindow(NULL, 0, 0, displayMode.w, displayMode.h, SDL_WINDOW_OPENGL | SDL_WINDOW_FULLSCREEN | SDL_WINDOW_RESIZABLE);
    if (!window) {
        printf("Could not initialize Windown");
        return 1;
    }
    auto gl = SDL_GL_CreateContext(window);
    if (!Init()) {
        cerr << "Error initializing OpenGL" << endl;
        return 1;
    }
    unique_ptr<Graphics> graphics = unique_ptr<Graphics>(new Graphics(window));
    SDL_iPhoneSetAnimationCallback(window, 1, UpdateFrame, graphics.get());
    SDL_AddEventWatch(EventFilter, NULL);

    //Game Loop
    SDL_Event event;
    auto done = false;
    while (!done)
    {
        SDL_PumpEvents();
        while (SDL_PollEvent(&event))
        {
            switch (event.type)
            {
                case SDL_QUIT:
                    done = true;
                    break;
                case SDL_APP_DIDENTERFOREGROUND:
                    SDL_Log("SDL_APP_DIDENTERFOREGROUND");
                    break;
                case SDL_APP_DIDENTERBACKGROUND:
                    SDL_Log("SDL_APP_DIDENTERBACKGROUND");
                    break;
                case SDL_APP_LOWMEMORY:
                    SDL_Log("SDL_APP_LOWMEMORY");
                    break;
                case SDL_APP_TERMINATING:
                    SDL_Log("SDL_APP_TERMINATING");
                    break;
                case SDL_APP_WILLENTERBACKGROUND:
                    SDL_Log("SDL_APP_WILLENTERBACKGROUND");
                    break;
                case SDL_APP_WILLENTERFOREGROUND:
                    SDL_Log("SDL_APP_WILLENTERFOREGROUND");
                    break;
                case SDL_WINDOWEVENT:
                {
                    switch (event.window.event)
                    {
                        case SDL_WINDOWEVENT_RESIZED:
                        {
                            SDL_Log("Window %d resized to %dx%d", event.window.windowID, event.window.data1, event.window.data2);
                            break;
                        }
                    }
                }
            }
        }
    }
    SDL_GL_DeleteContext(gl);
    // Done! Close the window, clean-up and exit the program.
    SDL_DestroyWindow(window);
    SDL_Quit();
    return 0;
}

也许我在这里错过了什么，任何帮助都将不胜感激。谢谢狭河道